It is known from British patent 716579 to make the flame tube of a jacketed jet tube heater of a number of hollow cylinderical ceramic parts disposed one behind the other and which are held together by interlocking ends of the hollow cylinderical ceramic parts as well as being cemented together. Such flame tubes are difficult to manufacture and are sensitive to unavoidable bending stresses that occur in operation.
In U.S. Pat. No. 2,478,732 a jacketed jet tube heater is disclosed in which a ceramic flame tube is likewise composed of pipe pieces or circular ring parts of which the terminal ring part adjacent to the end wall of the jacket tube is provided with radial passages for combustion product gas (hereinafter referred to as "exhaust gas") and, similarly, the other circular ring parts are provided on their outer surfaces outer surfaces with radial spacer ribs. These spacer ribs produce the centering of the flame tube in the radiant jacket tube and are for this purpose are supportive or held in place at their radially outer ends at the internal wall of the jacket tube. The circular ring parts, besides, are aligned flush to each other; they are pressed together tightly one against the other by springs acting against the free end of the column of circular ring parts which are supported by bearing against the end wall of the jacket tube. When this flame tube is used there remains a danger of local overheating of the radiant jacket tube in the regions of the exhaust gas channels discharging simply in the immediate neighborhood of the jacket tube end closure, whereas on the other hand, it is difficult to assure, at the butt joints of the flame tube's circular ring parts pressed together against each other only elastically, that there will be well defined flow relations and, likewise, to avoid that, in response to a bending stress of the flame tube, the abutments open up in a uncontrollable manner, with the result of producing local overheating of the radiant jacket tube.
In order to produce flame tubes disposed in jacket tubes with provision for flexibility required by bending stresses that arise in operation, it is also known from German patents 15 51 762 and 23 14 118 to produce flame tubes composed of segments consisting entirely or partly of ceramic materials. In these segments temperatures or slots forming exhaust gas channels through the material can be made. It is however also known to provide the exhaust gas channels at axis-parallel abutments of the circular ring segments with each other and to make these in such a way that they have a tangential direction component (German patent 23 14 118). For connecting pipe sections thus configured with each other overlapping muff connections (German patent 34 47 603) or bayonet joints can be used.
For increasing the service life of the jacket tube by which the heat output is radiated into the furnace, while taking into account NO.sub.x reducing precautions, a higher rate of circulation is necessary for hot exhaust gases flowing through the ring channel between the flame tube and the jacket tube, while at the same time a defined amount of heat transfer through the radiant jacket tube is to be produced with uniform temperature distribution over that tubes heat-delivering surface. The known ceramic radiant jacket tubes, because of the necessary flexibility, are composed of pieces of pipe or circular ring parts or segments. Either no definite flow relations are provided in the boundaries between parts abutting one upon the other or else the connection means used for the ceramic parts narrow down the already barely sufficient flow cross-section of the ring channel between the flame and jacket tubes and, moreover, additional material and expensive manufacturing methods are often required for these known flame tubes.